DOC: Add documentation on executable content options

[mgrover1]

Add docs on how to write:

• Python-based Jupyter Notebooks
• R-Markdown

[MathewBiddle]

@mgrover1 could you link to one of the examples you shared during HTH? I'm interested in using myst's capability for executable content in another project and would love a reference.

[mgrover1]

@MathewBiddle - here is a link to the myst docs directly!
https://mystmd.org/guide/notebooks-with-markdown